The present invention relates to poly(carbonate-co-urea) copolymers containing both carbonate —(O(C═O)O)— and urea —(N(C═O)N)— linkages and to a melt transesterification method of preparing these same copolymers. Although many methods of preparing polycarbonates are known, for example, solution, interfacial, melt transesterification, or solid state polymerization, polycarbonates are generally produced commercially through one of two types of processes: an interfacial process (so named because it involves reaction at a water/organic solvent interface) or a melt transesterification process (so named because the monomers are reacted in a molten, rather than dissolved, state). In the melt transesterification process, dihydroxy compounds such as bisphenol A are reacted with a carbonic acid diester. The carbonic acid diester is typically diphenyl carbonate. More recent patents have described using carbonic acid diesters that are more reactive than diphenyl carbonate, which are referred to herein as “activated” diaryl carbonates.
It is known from U.S. Pat. No. 4,336,182 to prepare polyurea (polycarbamide) and other nitrogen-containing homopolymers and homo-oligomers by a polymerization reaction of piperazine (six member aliphatic heterocyclic rings having two opposing secondary amine groups) or imadazolidine (5 member aliphatic heterocyclic rings containing a urea group) compounds in solution with carbonates, imidodicarbonates, diesters, and diisocyanate compounds. The disclosed method in U.S. Pat. No. 4,336,182 is limited to the preparation of polyurea homopolymers and homo-oligomers however. Therefore the polymers disclosed in U.S. Pat. No. 4,336,182 do not contain any carbonate linkages. Although such polyurea materials were reported to have useful anti-flaming and self-extinguishing properties, they apparently lacked many of the desirable properties of polycarbonates such as ductility and optical transparency. For example, most of the example reaction products in U.S. Pat. No. 4,336,182 were reported to be brittle and/or easily crushed, and none of the examples were reported to be transparent. In addition, solution polymerization methods are less desirable industrially for the preparation of polycarbonates due to the additional processing equipment and steps required to remove any residual organic solvent to the extremely low levels required so that such polymer properties as color, stability and mechanical properties are not negatively impacted.
It is known from U.S. Pat. No. 3,450,793 to prepare poly(carbonate-co-urethane) block copolymers containing urethane —(N(C═O)O)— groups in the form of polyurethane blocks. The method of U.S. Pat. No. 3,450,793 does not result in the preparation of poly(carbonate-co-urea) copolymers containing both carbonate —(O(C═O)O)— and urea —(N(C═O)N)— linkages however. In addition, the method of U.S. Pat. No. 3,450,793 actually requires two separate polymerization steps: one to make the polyurethane block, and a second polymerization to react the polyurethane blocks together with dihydroxy compounds and phosgene or bis-chloroformates or carbonic acid diesters.
Therefore there is still a need for poly(carbonate-co-urea) copolymers compositions containing both carbonate —(O(C═O)O)— and urea —(N(C═O)N)— linkages and to a melt transesterification method for preparing such copolymers.